As described in JP, B, 47-3927 and JP, B, 50-5354, for example, a prior art hydraulic drive system of the above-mentioned type comprises a variable displacement hydraulic pump, at least one actuator driven by a hydraulic fluid delivered from the hydraulic pump, a directional control valve of center bypass type, which has a meter-in passage provided with a meter-in variable restrictor and a center bypass passage provided with a bleed-off variable restrictor, for controlling a flow of the hydraulic fluid supplied from the hydraulic pump to the actuator, a low pressure circuit, a center bypass line for connecting the center bypass passage to the low pressure circuit at a location downstream of the bleed-off variable restrictor, a pressure generator, e.g., fixed restrictor, disposed in the center bypass line, and a pump regulator for controlling a displacement of the hydraulic pump by using, as a control pressure, the pressure generated by the fixed restrictor.
The pump regulator carries out well-known negative control in accordance with the control pressure generated by the fixed restrictor. More specifically, the pump regulator makes control such that the displacement of the hydraulic pump is increased as the control pressure lowers and the pump displacement is decreased as it rises.
In the prior art thus constructed, when the directional control valve is gradually stroked from its neutral position with an intention of driving the actuator, an opening area of the bleed-off variable restrictor of the directional control valve is gradually reduced and, to the contrary, an opening area of the meter-in variable restrictor thereof is gradually increased.
When the directional control valve is in its neutral position or at a start point of its stroke, i.e., when the bleed-off variable restrictor begins to close, the control pressure generated by the fixed restrictor is high and the hydraulic pump is kept at a predetermined small displacement to deliver the hydraulic fluid at a standby flow rate which is a small value corresponding to the above predetermined small displacement. As the bleed-off variable restrictor is gradually closed, a pressure of the hydraulic fluid delivered from the hydraulic pump, i.e., a pump pressure, rises. Assuming now that the load pressure of the actuator is Pa, the actuator starts moving at the time the pump pressure rises in excess of Pa. Upon the actuator starting to move and the hydraulic fluid from the hydraulic pump beginning to be supplied to the actuator, the flow rate passing through the center bypass passage is reduced accordingly. With this reduction in the flow rate passing through the center bypass passage, the control pressure generated by the fixed restrictor in the center bypass line is lowered. The pump regulator is thus driven in such a manner as to increase the displacement of the hydraulic pump. As a result, the delivery rate of the hydraulic pump is gradually increased, thereby providing a predetermined flow rate characteristic, i.e., metering characteristic.